1. Field of the Invention
This invention relates to mobile communications, and in particular to a method of controlling handover of a mobile station in a mobile communications network.
2. Description of the Related Technology
Handover algorithms are known for existing cellular wireless technologies. A cellular mobile station receiving service on uplink or downlink channels of a cell in a cellular network may experience worsening signal to noise (S/N) on the uplink and/or downlink channels, with the execution of a handover algorithm within the network resulting in a handover between channels in the cell or between different cells, to ensure a call is not dropped and to improve general quality of service during the call.
A number of different radio access technologies are proposed to be used in future to provide an appropriate level of service to the type of access a user may require at any particular time. The user's requirements may change from communications session to communications session or during a single communications session. To allow a user different types of access during a single communications session handover between the different radio access technologies may be desirable. For example, if a user requires a video conference link, a third generation radio access technology may be used. On the other hand, if only a voice call is desired, second generation radio access technologies may be sufficient. In the future heterogenous mobile environment, both nomadicity and migration of users should be supported. Thus, a user should be able to initiate a communications session using different radio access technologies and obtain delivery of a service while roaming between radio access technologies (nomadicity). Furthermore, handovers between radio access technologies should also be supported while the user is actively engaged in a communications session (migration). Examples of such handovers are between a second generation public GSM network, a third generation public Wideband Code-Division Multiple Access (W-CDMA) network and a wireless local area network (WLAN).